Stable measurement of sensors, methods and systems

ABSTRACT

Gain independent reference channel measurement system and method. A method of making robust, stable measurements, in a variety of different applications is disclosed. More specifically, this disclosure describes systems and methods relating to performing gain independent reference channel measurements by making two phase measurements of a device under test. Mathematically, the measurements are combined and many common mode parameters drop out. The result yields an analysis of a device under test analysis which mitigated errors, predominately arising from environmental variations and changes in circuit behavior stemming from swings in signal input.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of priority under35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/814,291entitled, “STABLE MEASUREMENT OF SENSORS METHODS AND SYSTEMS” filed onMar. 6, 2019 and related to U.S. Utility patent application Ser. No.16/181,878 entitled, “COMPACT OPTICAL SMOKE DETECTOR SYSTEM ANDAPPARATUS” filed on Nov. 6, 2018, U.S. Utility patent application Ser.No. 16/699,677 entitled, “FIRE DETECTION SYSTEM” filed on Dec. 1, 2019,U.S. Utility patent application Ser. No. 14/500,129 entitled, “LOWFREQUENCY NOISE IMPROVEMENT IN PLETHYSMOGRAPHY MEASUREMENT SYSTEMS”filed on Sep. 29, 2014, U.S. Utility patent application Ser. No.15/993,188 entitled, “COMPACT OPTICAL GAS DETECTION SYSTEM ANDAPPARATUS” filed on May 30, 2018, U.S. Provisional Patent ApplicationNo. 62/859,276 entitled, “GAS DETECTION USING DIFFERENTIAL PATH LENGTHMEASUREMENT” filed on Jun. 10, 2019, all of which are herebyincorporated by reference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a method of making stablemeasurements, in a variety of different applications. More specifically,this disclosure describes systems and methods relating to performinggain independent reference channel measurements and applicationsthereto.

BACKGROUND

Reference channel measurements are system measurements performed inparallel with the measurement of a device under test (DUT). Referencechannel measurements serve as a control measurement to be used incomparison with device under test (DUT) measurement. In theoryenvironmental changes and signal variation is supposed to affect the twomeasurement pathways equally. To this end, differences between thedevice under test (DUT) pathway and the Reference channel measurementpathway should solely reflect a change in stimuli. Accordingly, circuitscan be calibrated after assembly to compensate for measurement pathwaydisparities.

However, the inventor of the present disclosure has found this notentirely, particularly in attempting sensitive measurement.Specifically, environmental changes and signal variations (e.g.,nonlinearities resulting from high current, etc.) can affect respectivegain amplifiers differently. Consequently, the accuracy of the deviceunder test (DUT) measurement can be compromised, sometimes on the orderof magnitudes. Accordingly, there is a long felt need for a Referencechannel measurement which can be implemented with only nominalarchitecture modifications.

The inventor of the present disclosure has identified these shortcomingsand recognized a need for a new reference channel measurement techniquethat is more stable and superior to the traditional reference channelmeasurements. That is, a robust reference channel measurement which notonly accounts for common mode changes but gain differences betweenamplifiers.

This disclosure is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. Furtherlimitations and disadvantages of conventional and traditional approacheswill become apparent to one of skill in the art, through comparison ofsuch systems with some aspects of the present invention as set forth inthe remainder of the present application with reference to the drawings.

SUMMARY OF THE DISCLOSURE

Gain independent reference channel measurement system and method. Amethod of making robust, stable measurements, in a variety of differentapplications is disclosed. More specifically, this disclosure describessystems and methods relating to performing gain independent referencechannel measurements by making two phase measurements of a device undertest. Mathematically, the measurements are combined and many common modeparameters drop out. The result yields an analysis of a device undertest analysis which mitigated errors, predominately arising fromenvironmental variations and changes in circuit behavior stemming fromswings in signal input.

According to one aspect of the present disclosure is an apparatus for ananalog signal processing circuit having at least two amplifiersconnected by a switch that allows these amplifiers to connect to eitherinputs in succession.

According to another aspect, these measurements of the two sensors arecarried out in succession such that in the first instance firstamplifier is connected to the first sensor and the second amplifier isconnected to the second sensor. While according to the second instance,the first amplifier is connected to the second sensor and the secondamplifier is connected to the first sensor. These measurements are thencombined to provide measurements that only depend on the relative sensorresponses and not on the amplifier gains.

According to one aspect of the present disclosure, an analog front end(AFE) is used as the analog signal processing circuit.

According to one or more aspects of the present disclosure, the AFE isused to measure plethysmography (PPG) signals where the two photodiodesare placed at two different distances and both photodiodes measure lighthaving passed thru the tissue.

According to one or more aspects of the present disclosure, the AFE isused to measure PPG signals where the two photodiodes are placed suchthat one photodiode directly monitors the LED while the other measureslight having passed thru the tissue.

According to one or more aspects of the present disclosure, the AFE isused to measure optical signals where LED or stimulus light source ismonitored using its current via electrical measurement while the opticalsignal is measured by the photodiode.

According to one or more aspects of the present disclosure, the AFE isused to measure impedances in which direct measurement of test impedanceis measured relative to the standard or reference impedance.

According to one or more aspects of the present disclosure, the AFE isused to measure attenuation of light from smoke and other scatteringparticles placed between the light source and detector.

According to one or more aspects of the present disclosure, the AFE isused to measure variation in light due to changes in distance from thelight source to the detector.

According to one or more aspects of the present disclosure, the AFE isused to measure variation in absorption due to varying concentration ofabsorber in which the sense channel and reference channel are placed attwo different distances from the light source.

The drawings show exemplary stable, robust method for measuring sensorsin a manifold of applications and configurations thereof. Variations ofthese circuits, for example, changing the positions of, adding, orremoving certain elements from the circuits are not beyond the scope ofthe present invention. The illustrated stable measurement circuitdevices and configurations are intended to be complementary to thesupport found in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not necessarily drawn to scale, and are used forillustration purposes only. Where a scale is shown, explicitly orimplicitly, it provides only one illustrative example. In otherembodiments, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

For a fuller understanding of the nature and advantages of the presentinvention, reference is made to the following detailed description ofpreferred embodiments and in connection with the accompanying drawings,in which:

FIG. 1 shows an exemplary optical reference channel monitoring circuitin a first mode, in accordance with some embodiments of the disclosureprovided herein;

FIG. 2 shows an exemplary optical reference channel monitoring circuitin another mode, in accordance with some embodiments of the disclosureprovided herein;

FIG. 3 demonstrates an exemplary reference channel monitoring circuit inan impedance variation (complex or otherwise), in accordance with someembodiments of the disclosure provided herein;

FIG. 4 demonstrates an exemplary reference channel monitoring circuitwithin a mixed reference channel, wherein the sense measurement signalis optical while the reference signal is electrical, in accordance withsome embodiments of the disclosure provided herein;

FIG. 5 illustrates an exemplary optical reference channel monitoringcircuit within the context of another application, in accordance withsome embodiments of the disclosure provided herein;

FIG. 6 illustrates an exemplary optical reference channel monitoringcircuit within the application of yet another application, in accordancewith some embodiments of the disclosure provided herein;

FIG. 7 shows a side view of an exemplary optical gas detectionmeasurement system, in accordance with some embodiments of thedisclosure provided herein; and

FIG. 8 depicts the side view of an exemplary differential path lengthgas detection system, in accordance with some embodiments of thedisclosure provided herein.

DETAILED DESCRIPTION

The present disclosure relates to a method of making stablemeasurements, in a variety of different applications. More specifically,this disclosure describes systems and methods relating to performinggain independent reference channel measurements and applicationsthereto.

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrative examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure are set forthin the proceeding in view of the drawings where applicable.

A new reference channel measurement technique that is more stable andsuperior to the traditional reference channel measurement is disclosed.Many sense modalities require stimulation of the device under test (DUT)with light or electromagnetic radiation or electrical voltage or currentand measure the response. The changes in the response represents thechanges in the environment of the sensor. Example includes opticalmeasurement of changes in transmission or reflection due to underlyingmaterial property changes, changes in impedance of the material as afunction of temperature and or some other environmental parameters.

This list of applications is numerous. In many of these instances, it isvery important to ensure that the measured value does not depend on thevariation in the gain of the electronic signal processing or changes inthe intensity of stimulus due to varying temperature or voltage.

The traditional method for making these measurements in more stablefashion is to use a reference channel. This will now be discussed in thecontext of the present disclosure. FIG. 1 shows an exemplary opticalreference channel monitoring circuit 100 in a first mode, in accordancewith some embodiments of the disclosure provided herein.

Optical reference channel monitoring circuit 100 comprises lightemitting diode (LED) 105, function generator 110, device under test(DUT) 125, sense detector 135, reference detector 140, sense amplifier150, and reference amplifier 145.

Function generator 110 is a piece of electronic test equipment orsoftware used to generate different types of electrical waveforms over awide range of frequencies. Some of the most common waveforms produced bythe function generator are the sine wave, square wave, triangular waveand sawtooth shapes. These waveforms can be either repetitive orsingle-shot (which requires an internal or external trigger source). Inone or more embodiments, function generator 110 is current sourceconfigured to produce a square-wave pulse 155. However, any type ofcurrent and/or voltage source is not beyond the scope of the presentdisclosure. Furthermore, there is no necessity to produce particularsignals. Reproducibility is rather an objective to achieve the desiredresults.

Current from function generator 110 passes through light emitting diode(LED) 105. In turn, light emitting diode (LED) 105 produces light, someof which (light 105) impinges on device under test (DUT) 125 while otherlight 120 is directly received at reference detector 140. Thespecificity of device under test (DUT) 125 depends on the application.Some of the applications will be discussed in greater detail later inthe disclosure. For the present exemplary embodiment, device under test(DUT) 125 is an abstraction whose light output 130 depends on thereceived light 105.

Sense detector 135 receives light 130. In one or embodiments, sensedetector 135 and reference detector 140 are photodetectors.Photodetectors are sensors of light or other electromagnetic energy.Photodetectors have p-n junctions that converts light photons intocurrent. The absorbed photons make electron-hole pairs in the depletionregion, which is used to detect received light intensity. In someembodiments, photodetectors are photodiodes or phototransistors.However, any light detecting means, e.g., avalanche, photo-multipliertube, etc., is not beyond the scope of the present disclosure.

In one or more embodiments, LED 105 is an off-the-shelf green (495nm-570 nm) light emitting diode. However, any suitable compact lightproducing device is not beyond the scope of the presentdisclosure—whether coherent, incandescent, or even thermal black-bodyradiation, etc.

In the present embodiment, the stimulus or light emitted 115, 120 from alight source such as LED 105 is measured in two ways. Part of thestimulus (light) is measured after interacting with the DUT 125 formingthe sense channel at the sense detector 135. Meanwhile at the referencedetector 140, the same stimulus (light 120) is measured directly forminga reference channel.

The two measurements may be written as:

M_(s)=η_(l)I_(l)H_(dut)R_(s)G_(s)

M_(R)=η_(l)I_(l)H_(R)R_(R)G_(R)  (1)

Here I_(l) is the current stimulus which is converted to light withefficiency η_(l) which then interacts with the DUT transforming thestimulus by H_(dut) and then converted back to electrical domain withdetector of responsivity R_(s) and then processed by electronic circuitwith gain G. Similar measurement is done with the reference channel.Clearly taking ratio makes it independent of the variation in thestimulus itself. This is shown below:

$\begin{matrix}{\rho_{traditional} = {\frac{M_{S}}{M_{R}} = {\frac{\eta_{l}l_{l}H_{dut}R_{S}G_{S}}{\eta_{l}l_{l}H_{R}R_{R}G_{R}} = {\frac{H_{dut}R_{S}G_{S}}{H_{R}R_{R}G_{R}} = {( \frac{H_{dut}}{H_{R}} )( \frac{R_{S}}{R_{R}} )( \frac{G_{S}}{G_{R}} )}}}}} & (2)\end{matrix}$

Since the measurement of interest is the changes in the first term inthe bracket, i.e., measurement of H_(dut), the variation of the othertwo bracketed terms must be minimized or eliminated. The presentdisclosure presents a method to eliminate the last bracket and make themeasurement independent of the ratio

$\frac{G_{S}}{G_{R}},$

which will now be discussed in greater detail.

FIG. 2 shows an exemplary optical reference channel monitoring circuitin another mode, in accordance with some embodiments of the disclosureprovided herein. Optical reference channel monitoring circuit 200comprises light emitting diode (LED) 205, function generator 210, deviceunder test (DUT) 225, sense detector 235, reference detector 240, switchfabric 260, sense amplifier 250, and reference amplifier 245.

Similarly, to previously described, function generator 210 is a piece ofelectronic test equipment or software used to generate different typesof electrical waveforms over a wide range of frequencies. Some of themost common waveforms produced by the function generator are the sinewave, square wave, triangular wave and sawtooth shapes. These waveformscan be either repetitive or single-shot (which requires an internal orexternal trigger source). In one or more embodiments, function generator210 is current source configured to produce a square-wave pulse 155.However, any type of current and/or voltage source is not beyond thescope of the present disclosure. Furthermore, there is no necessity toproduce particular signals. Reproducibility is rather an objective toachieve the desired results.

Current from function generator 210 passes through light emitting diode(LED) 205. In turn, light emitting diode (LED) 205 produces light, someof which (light 205) impinges on device under test (DUT) 225 while otherlight 220 is directly received at reference detector 240. Thespecificity of device under test (DUT) 225 depends on the application.Some of the applications will be discussed in greater detail later inthe disclosure. For the present exemplary embodiment, device under test(DUT) 225 is an abstraction whose light output 230 depends on thereceived light 205.

In one or more embodiments, switch fabric 260 comprises one or moretransistors configured to cross switch at a predetermined time, whichmay or may not be coordinated with the input signal 255. A transistor isa semiconductor device used to amplify or switch electronic signals andelectrical power. It is composed of semiconductor material usually withat least three terminals for connection to an external circuit. Avoltage or current applied to one pair of the transistor's terminalscontrols the current through another pair of terminals.

Transistors which can be used include metal-oxide-semiconductorfield-effect transistors (MOSFETs) and bipolar junction transistors(bipolar transistor or BJTs) which is a type of transistor that usesboth electrons and holes as charge carriers. MOSFETs are a type ofinsulated-gate field-effect transistor (IGFET) that are fabricated bythe controlled oxidation of a semiconductor, typically silicon. However,any transistor or digital circuit is not beyond the scope of the presentdisclosure.

In some embodiments, switch fabric 260 is a switching circuit which isknown in the art. In other embodiments, switch fabric 260 is an analogmultipole relay. In practice, switch fabric 260 directs the signalreceived from sense detector 235 to either sense amplifier 250 orreference amplifier 245. Similarly, switch fabric 260 directs the signalreceived from reference detector 240 to either sense amplifier 250 orreference amplifier 245. The importance of cross coupling will bediscussed later in the disclosure.

Sense detector 235 receives light 230. In one or more embodiments, sensedetector 235 and reference detector 240 are photodetectors.Photodetectors are sensors of light or other electromagnetic energy.Photodetectors have p-n junctions that converts light photons intocurrent. The absorbed photons make electron-hole pairs in the depletionregion, which is used to detect received light intensity. In someembodiments, photodetectors are photodiodes or phototransistors.However, any light detecting means, e.g., avalanche, photo-multipliertube, etc., is not beyond the scope of the present disclosure.

In the present embodiment, the stimulus or light emitted 215, 220 from alight source such as LED 205 is measured in two ways. Part of thestimulus (light) is measured after interacting with the DUT 225 formingthe sense channel at the sense detector 235. Meanwhile at the referencedetector 240, the same stimulus (light 220) is measured directly forminga reference channel.

To illustrate in practice, an optical measurement embodiment isexemplified, but the idea applies more generally. In broadestperspective, this is accomplished by at least two-phase measurement todivide out all common mode changes in the path of the measurement systemincluding the variations in the signal conditioning circuit (SCC). FIG.2 expands on FIG. 1 and illustrates how this is accomplished.

This is done in two steps. In the first phase of the measurementsequence, switch connects sense channel to amplifier with gain G_(A) andthe reference channel is connected to amplifier with gain G_(B). Fromabove equation 1 is rewritten as follows:

M_(s1)=η_(l)I_(l)H_(dut)R_(s)G_(A)

M_(R1)=η_(l)I_(l)H_(R)R_(R)G_(B)  (3)

In the second phase of the measurement, stimulus is presented again butthe switch is moved to the crossed position such that sense channel isnow connected to amplifier with gain G_(B) and the reference channel toG_(A). This yields:

M_(s2)=η_(l)I_(l)H_(dut)R_(s)G_(B)

M_(R2)=η_(l)I_(l)H_(R)R_(R)G_(A)  (4)

From these two measurements, a combination can be formed to eliminateboth variation in stimulus as in traditional reference channel-basedmeasurement but also gains of the amplifier. This is done by:

$\begin{matrix}{\rho = {\frac{M_{S1} + M_{S2}}{M_{R1} + M_{R2}} = {\frac{{\eta_{l}l_{l}H_{dut}R_{s}G_{A}} + {\eta_{l}l_{l}H_{dut}R_{s}G_{B}}}{{\eta_{l}l_{l}H_{R}R_{R}G_{B}} + {\eta_{l}l_{l}H_{R}R_{R}G_{A}}} = {( \frac{H_{dut}}{H_{R}} )( \frac{R_{S}}{R_{R}} )}}}} & (5)\end{matrix}$

This simple operation creates enormous advantages in the signalmeasurement as it lifts the burden of very high gain stability from thesignal processing AFE. In general, the two phases of measurement arecarried out in rapid succession. This method even suppresses to a highdegree even the changes in the gains between the two measurements.Suppose δG is the small change in the gains of the amplifiers betweenthe two successive measurements. Then simple algebra will show that theerror in ρ is:

$\rho = {( \frac{H_{dut}}{H_{R}} )( \frac{R_{S}}{R_{R}} )( {1 + {\delta {G( \frac{G_{A} - G_{B}}{G_{A} + G_{B}} )}}} )}$

The error term is likely to be less than 10⁻⁵ since changes in gain δGin a short time between phases are likely to be <10⁻³ and for anominally matched gains, the term

$( \frac{G_{A} - G_{B}}{G_{A} + G_{B}} )$

is also likely to be less than 1%. This shows that this measurementmethod is highly robust.

Even though reference channel measurements are very common and areusually done at the system level or board level or even at theinstrument level, a novel approach is provided as a new built-intechnique to the circuit architecture that allows a far better and moreefficient reference channel measurement. It eliminates concerns aroundthe long-term drift of the amplifiers and circuits that allows for usein systems that need long-term stability such as smoke detectors, gasabsorption sensors, optical tissue measurements, sensors that changeimpedance in response to environment such as temperature, humidity etc.

As a matter of practice, some of the terms in equation 5 can becalibrated out by providing a standard H_(dut) ⁰ at the time ofcalibration. Then the new ratio:

$\begin{matrix}{\frac{\rho}{\rho_{0}} = {{( \frac{H_{dut}}{H_{R}} ){( \frac{R_{S}}{R_{R}} )/( \frac{H_{dut}^{0}}{H_{R}} )}( \frac{R_{S}}{R_{R}} )} = \frac{H_{dut}}{H_{dut}^{0}}}} & (6)\end{matrix}$

Thus, this method makes it convenient to directly compare themeasurement to the standard.

This method can be applied more widely than the previous embodimentsabove. FIG. 3 is an application of this method to the measurement ofresistance (or impedance) to a very high precision and stability iselucidated.

FIG. 3 demonstrates an exemplary reference channel monitoring circuit300 in an impedance variation (complex or otherwise), in accordance withsome embodiments of the disclosure provided herein. Reference channelmonitoring circuit 300 comprises input node 365, sense impedance 370,reference impedance 375, switch fabric 360, sense amplifier 350, andreference amplifier 345.

In one or more embodiments, switch fabric 360 comprises one or moretransistors configured to cross switch at a predetermined time, whichmay or may not be coordinated with the input signal 355. Transistorswhich can be used include metal-oxide-semiconductor field-effecttransistor (MOSFET) and bipolar junction transistor (bipolar transistoror BJT) which is a type of transistor that uses both electrons and holesas charge carriers.

In some embodiments, switch fabric 360 is a switching circuit which isknown in the art. In other embodiments, switch fabric 360 is an analogmultipole relay. In practice, switch fabric 360 directs the signalreceived from sense detector 335 to either sense amplifier 350 orreference amplifier 345. Similarly, switch fabric 360 directs the signalreceived from reference detector 340 to either sense amplifier 350 orreference amplifier 345.

In this case, light source is replaced by a voltage stimulus over node365, and current is directly measured. In this case, the ratio isdirectly:

$\begin{matrix}{\rho = {{( \frac{v}{Z_{R}} )/( \frac{v}{Z_{S}} )} = \frac{Z_{S}}{Z_{R}}}} & (7)\end{matrix}$

In FIG. 4, a “mixed” reference channel measurement is demonstrated inimplantation and practice. The sense measurement is optical while thereference channel is electrical. FIG. 4 demonstrates an exemplaryreference channel monitoring circuit 400 within a mixed referencechannel, wherein the sense measurement signal is optical while thereference signal is electrical, in accordance with some embodiments ofthe disclosure provided herein.

Electrical reference channel monitoring circuit 400 comprises lightemitting diode (LED) 405, function generator 410, device under test(DUT) 425, series resistor 480, sense detector 435, reference circuit(capacitors 485 and reference resistors 490) switch fabric 460, senseamplifier 450, and reference amplifier 445.

A small series resistor 480 is added in series with the LED (or laser)405 and the voltage (V_(ref)) is measured across it, thereby measuringthe current. FIG. 4 shows this resistor 480 near the voltage supply (notshown) to the LED 405 and then this small voltage is “converted” back tocurrent input via DC blocking capacitors 485 and series referenceresistors 490.

Thus, this can be connected to the same amplifier as the photodiodeinput amplifier via switch fabric 460. The sense resistor r_(led) 480can also be placed near ground. In this case of FIG. 4, the referencechannel will “cancel” the variation in the driver current but not thosedue to the LED 405 itself. The ratio is approximately given by (moreexact expression including capacitance etc. can be easily written butthe basic idea is easily illustrated by ignoring capacitor c_(s)):

$\begin{matrix}{\rho = {( \frac{\frac{\eta_{l}}{r_{led}}}{r_{S}} )( H_{dut} )( R_{S} )}} & (8)\end{matrix}$

Some practical, non-exhaustive applications of this idea will now bediscussed.

Application 1: Ultra-Stable Intensity Measurement in Reflection,Transmission, Absorption, and Scattering

This is illustrated with smoke measurement in FIG. 5. FIG. 5 illustratesan exemplary optical reference channel monitoring circuit within thecontext of another application, in accordance with some embodiments ofthe disclosure provided herein.

Reference channel monitoring circuit 500 comprises light emitting diode(LED) 505, function generator 510, sense detector 535, referencedetector 540, and blocking elements 585. One skilled in the art willappreciate that some circuit elements have been omitted but theprinciple remains same as one or more of the previous embodiments.

In practice, some light 575 produced from light emitting diode (LED) 505passes through an aperture which is defined by blocking elements 585.Some light 575 gets scattered 590 off of smoke particles 595. And, somelight 575 proceeds unimpeded and is received at sense detector 535. Oneor ordinary skill will recognize the present implementation as anobscuration sensor. Other light 520 produced from light emitting diode(LED) 505 is incident upon reference detector 540.

Smoke particles 595 between detector 535 and LED 505 cause reduction inlight intensity by scattering light 590. Typical smoke alarm thresholdsmay be as low as 1%/ft—or smoke causes 1% light attenuation withone-foot path length. A 1 cm path length—which makes for a very smalland compact device—implies that we need to measure changes at 10⁻⁴level. That is not very hard—to maintain SNR of ˜80 dB. What isdifficult is to maintain the stability of the amplifiers, LED etc. atthis level so that heat and other environmental parameters do not causemeasurement to shift. Again, the technique demonstrated makes suchmeasurement possible in a compact sensor.

This stability can be used to measure small changes in distance by usingintensity and the fact that intensity falls off as a function ofdistance. Again, this requires that variations from all otherenvironmental parameters get suppressed.

Application 2: PPG Measurements

A photoplethysmogram (PPG) is an optically obtained plethysmogram thatcan be used to detect blood volume changes in the microvascular bed oftissue. A PPG is often obtained by using a pulse oximeter whichilluminates the skin and measures changes in light absorption. Aconventional pulse oximeter monitors the perfusion of blood to thedermis and subcutaneous tissue of the skin.

FIG. 6 illustrates an exemplary optical reference channel monitoringcircuit within the application of yet another application, in accordancewith some embodiments of the disclosure provided herein. Referencechannel monitoring circuit 600 comprises light emitting diode (LED) 605,sense detector 635, and reference detector 640. One skilled in the artwill appreciate that some circuit elements have been omitted but thatthe principle remains the same as one or more of the previousembodiments.

In practice, light emitting diode (LED) 605 produces light 620 which inturn get scattered off of a predetermined chemical, e.g., SpO₂, withinthe tissue of a subject (patient). Subsequently scatter light 690 getsdetected by either reference detector 640 or sense detector 635,depending on scattering trajectory and mean-free-path. This is afunction of the light wavelength and chemical interaction which is knownin the art.

In one or more embodiments, a separate photodiode may be deployed to actas reference channel to eliminate low frequency variations in the LED'soutput due to temperature and supply variations. Since a heart beats atroughly 1 Hz, this low-frequency elimination of LED's variation as wellas any variation in gains allows one to reach high SNR even with noisypower supplies that generally add lots of noise and systematicvariations at low frequencies.

In yet another embodiment associated with FIG. 6, two photodiodes can beused with one closer to the LED designated as the reference PD while theone further away acting as signal PD.

In this case, even the variation in the LED's light coupling to thetissue becomes common mode and are eliminated. This will allow moreprecise measurement of the tissue scattering and absorption.

Application 3: Gas Absorption Measurement

FIG. 7 shows a side view of an exemplary optical gas detectionmeasurement system 700, in accordance with some embodiments of thedisclosure provided herein. Optical gas detection measurement system 700comprises substrate 720, LED 710, reflective surface 780, cap 760, cover970, photodetector 740, photodetector 750 and septum 730.

In practice, gas detection is performed as follows. LED 750 produceslight, some of which enters the gas chamber defined by reflectivesurface 780. Another portion of light directly impinges on photodetector740 which act as the previously described reference channel, at least inpart. Cap 760 is a packaging choice. In one or more embodiments, cap 760includes a gas chamber defined by the boundaries of reflective surface780. Typically, gas chamber comprises ingress and egress apertures (notshown) to allow for the passage of gas though the chamber.

In some embodiments, cap 760 is three-dimensional conic-section shapedsurface, such as, an ellipsoid or paraboloid. However, other shapes arenot beyond the scope of the present invention. For example, twodimensional conic sections (as viewed from the side as shown) can bealmost as effective in detection. Further still, one side can be asimple plane oriented at 45-degree angle in relation to photodetector750. In practice, light will reflect twice on average while traversingthrough the gas chamber before being detected by photodetector 750.

Septum 730 is disposed between LED 710 and photodetector 750 such thatlight does not pass directly to the photodetector 750. In the presentembodiment, photodetector 750 is the aforementioned sense detector, asleast in part. In one or more embodiments, cover 770 can be used tosimplify packaging. In others, cover 770 can be optical filters whichare known in the art.

FIG. 8 depicts the side view of an exemplary differential path lengthgas detection system 800, in accordance with some embodiments of thedisclosure provided herein. Optical differential gas detectionmeasurement system 800 comprises substrate, LED 810, reflective surfaces880, 890, photodetector 840, photodetector 850 and septum 830.

In practice, differential gas detection is performed as follows. LED 850produces light, primary path 860 and reference path 870, which entersinto gas chamber. Similar to that previously described, gas chamber isdefined by the boundaries of reflective surfaces 880, 890. Typically,gas chamber comprises ingress and egress apertures (not shown) to allowfor the passage of gas though the chamber.

In some embodiments, reflective surfaces 880, 890 can bethree-dimensional conic-section shaped surfaces, such as, an ellipsoidsor paraboloids. However, other shapes are not beyond the scope of thepresent invention. For example, two dimensional conic sections (asviewed from the side as shown) can be almost as effective in detection.Further still, one side can be a simple plane oriented at 45-degreeangle in relation to photodetector 850. In practice, light (primary path860) will reflect twice on average while traversing through the gaschamber before being detected by photodetector 850. Whereas, referencepath 870 will only reflect once before being detected by referencedetector 840.

Septum 830 is disposed to prevent direct illumination of photoreactors840, 850. In the present embodiment, photodetector 850 is theaforementioned sense detector and photodetector 840 act of the referencedetector, as least in part, signal and amplifiers notwithstanding. Insome embodiments, optical filters which are known in the art can bedisposed proximal to LED 810.

This is illustrated in our differential absorption measurementapplication. One of ordinary skill in the art will appreciate thatreference path 870 will inherently have a shorter pathlength thanprimary path 860. It is the known (or determined) pathlength differencewhich is used to calculate optical absorption of a predetermined gaswith a corresponding wavelength spectrum.

Select Examples

Example 1 provides an apparatus for making gain independent referencechannel measurements, the apparatus comprising a first circuitconfigured to measure a stimulus passing through a testing object, afirst amplifier, a second circuit configured to measure a stimulus whichhas not passed through the test volume, a second amplifier, and aswitching circuit in electrical communication with the first circuit,second circuit, first amplifier and second amplifier.

In the first example, the switching circuit configured to change betweena first mode wherein first circuit is in electrical communication withthe first amplifier and the second circuit is in electricalcommunication with the second amplifier, and a second mode wherein firstcircuit is in electrical communication with the second amplifier and thesecond circuit is in electrical communication with the first amplifier.

Example 2 provides an apparatus according to example 1 wherein the firstcircuit comprises a sensor.

Example 3 provides an apparatus according to examples 1-2 wherein thesensor is a photodetector.

Example 4 provides an apparatus according to any one or more of thepreceding examples, wherein the second circuit comprises aphotodetector.

Example 5 provides an apparatus according to any one or more of thepreceding examples further comprising a light source which produces thestimulus.

Example 6 provides an apparatus according to example 5, wherein thelight source is an LED.

Example 7 provides an apparatus according to any one or more of thepreceding examples, further comprising a current source.

Example 8 provides an apparatus according to example 7, wherein thecurrent source is configured to produce a predetermined waveform in boththe first and second mode.

Example 9 provides an apparatus according to any one or more of thepreceding examples further comprising an analog front end (AFE)configured to compare output from the first and second gain amplifiersin the first and second modes.

Example 10 provides an apparatus according to any one or more of thepreceding examples, wherein the first circuit comprises a transducer.

Example 11 provides an apparatus according to any one or more of thepreceding examples, wherein the testing object is a device under testwhich has an impedance to be measure by the stimuli.

Example 12 provides an apparatus according to any one or more of thepreceding examples, wherein the testing object is a testing volume.

Example 13 provides an apparatus according to any one or more of thepreceding examples, wherein the testing object is a subject.

Example 14 provides a method for making gain independent referencechannel measurements comprising measuring at a first circuit a stimuluspassing through a testing object, measuring at a second circuit astimulus which has not passed through the test volume, receiving at aswitch circuit the measurements from the first and second circuits, in afirst mode, outputting from the switch circuit the first and secondmeasurement to a first and second amplifier, respectively, switching theswitch circuit, and in a second mode, outputting from the switch circuitthe first and second measurement to the second and first amplifier,respectively.

Example 15 provides a method according to example 14 further comprisingamplifying the first and second measurement in the first and secondamplifiers, respectively, during the first mode.

Example 16 provides a method according to any one or more of thepreceding examples further comprising amplifying the first and secondmeasurement in the second and first amplifiers, respectively, during thesecond mode.

Example 17 provides a method according to any one or more of thepreceding examples further comprising calculating a ratio based on atleast amplified measurements in during the first and second modes.

Example 18 provides a method according to any one or more of thepreceding examples further comprising determining the presence of apredetermined species based on the calculated ratio.

Example 19 provides a method according to example 18, wherein thespecies are smoke particles.

Example 20 provides a method according to example 18, wherein thespecies is a biochemical.

Example 21 provides a method according to example 18, wherein thespecies is an inorganic chemical.

Example 22 provides a method according to example 18, wherein thespecies is an organic chemical.

Example 23 provides a method according to any one or more of thepreceding examples, wherein the first circuit comprises a sensor.

Example 24 provides a method according to example 23 wherein the sensoris a photodetector.

Example 25 provides a method according to any one or more of thepreceding examples, wherein the second circuit comprises aphotodetector.

Example 26 provides a method according to any one or more of thepreceding examples further comprising illuminating a light source.

Example 27 provides a method according to example 26 wherein the lightsource is and LED.

Example 28 provides a method according to example 27 further comprisingproviding a current through the LED

Example 29 provides a method according to any one or more of thepreceding examples further comprising producing a predetermined waveformduring both the first and second mode.

Example 30 provides a method according to example 17 wherein the ratiois calculated using an analog front end (AFE) configured to compareoutput from the first and second gain amplifiers in the first and secondmodes.

Example 31 provides a method according to any one or more of thepreceding examples, wherein the testing object is a testing volume.

Example 32 provides a method according to any one or more of thepreceding examples, wherein the testing object is a subject.

Example 33 provides for an apparatus for making gain independentreference channel measurements comprising a means for measuring at afirst circuit a stimulus passing through a testing object, a means formeasuring at a second circuit a stimulus which has not passed throughthe test volume, a means for receiving at a switch circuit themeasurements from the first and second circuits, in a first mode, ameans for outputting from the switch circuit the first and secondmeasurement to a first and second amplifier, respectively, a means forswitching the switch circuit; and in a second mode, a means foroutputting from the switch circuit the first and second measurement tothe second and first amplifier, respectively.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments described herein. It is, therefore, to be understood thatthe foregoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, kits, and/or methods described herein, ifsuch features, systems, articles, materials, kits, and/or methods arenot mutually inconsistent, is included within the scope of the presentdisclosure.

The foregoing outlines features of one or more embodiments of thesubject matter disclosed herein. These embodiments are provided toenable a person having ordinary skill in the art (PHOSITA) to betterunderstand various aspects of the present disclosure. Certainwell-understood terms, as well as underlying technologies and/orstandards may be referenced without being described in detail. It isanticipated that the PHOSITA will possess or have access to backgroundknowledge or information in those technologies and standards sufficientto practice the teachings of the present disclosure.

The PHOSITA will appreciate that they may readily use the presentdisclosure as a basis for designing or modifying other processes,structures, or variations for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein. ThePHOSITA will also recognize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions, and alterations hereinwithout departing from the spirit and scope of the present disclosure.

The above-described embodiments may be implemented in any of numerousways. One or more aspects and embodiments of the present applicationinvolving the performance of processes or methods may utilize programinstructions executable by a device (e.g., a computer, a processor, orother device) to perform, or control performance of, the processes ormethods.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other tangible computer storage medium) encoded with one ormore programs that, when executed on one or more computers or otherprocessors, perform methods that implement one or more of the variousembodiments described above.

The computer readable medium or media may be transportable, such thatthe program or programs stored thereon may be loaded onto one or moredifferent computers or other processors to implement various ones of theaspects described above. In some embodiments, computer readable mediamay be non-transitory media.

Note that the activities discussed above with reference to the FIGURESwhich are applicable to any integrated circuit that involves signalprocessing (for example, gesture signal processing, video signalprocessing, audio signal processing, analog-to-digital conversion,digital-to-analog conversion), particularly those that can executespecialized software programs or algorithms, some of which may beassociated with processing digitized real-time data.

In some cases, the teachings of the present disclosure may be encodedinto one or more tangible, non-transitory computer-readable mediumshaving stored thereon executable instructions that, when executed,instruct a programmable device (such as a processor or DSP) to performthe methods or functions disclosed herein. In cases where the teachingsherein are embodied at least partly in a hardware device (such as anASIC, IP block, or SoC), a non-transitory medium could include ahardware device hardware-programmed with logic to perform the methods orfunctions disclosed herein. The teachings could also be practiced in theform of Register Transfer Level (RTL) or other hardware descriptionlanguage such as VHDL or Verilog, which can be used to program afabrication process to produce the hardware elements disclosed.

In example implementations, at least some portions of the processingactivities outlined herein may also be implemented in software. In someembodiments, one or more of these features may be implemented inhardware provided external to the elements of the disclosed figures, orconsolidated in any appropriate manner to achieve the intendedfunctionality. The various components may include software (orreciprocating software) that can coordinate in order to achieve theoperations as outlined herein. In still other embodiments, theseelements may include any suitable algorithms, hardware, software,components, modules, interfaces, or objects that facilitate theoperations thereof.

Any suitably-configured processor component can execute any type ofinstructions associated with the data to achieve the operations detailedherein. Any processor disclosed herein could transform an element or anarticle (for example, data) from one state or thing to another state orthing. In another example, some activities outlined herein may beimplemented with fixed logic or programmable logic (for example,software and/or computer instructions executed by a processor) and theelements identified herein could be some type of a programmableprocessor, programmable digital logic (for example, an FPGA, an erasableprogrammable read only memory (EPROM), an electrically erasableprogrammable read only memory (EEPROM)), an ASIC that includes digitallogic, software, code, electronic instructions, flash memory, opticaldisks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types ofmachine-readable mediums suitable for storing electronic instructions,or any suitable combination thereof.

In operation, processors may store information in any suitable type ofnon-transitory storage medium (for example, random access memory (RAM),read only memory (ROM), FPGA, EPROM, electrically erasable programmableROM (EEPROM), etc.), software, hardware, or in any other suitablecomponent, device, element, or object where appropriate and based onparticular needs. Further, the information being tracked, sent,received, or stored in a processor could be provided in any database,register, table, cache, queue, control list, or storage structure, basedon particular needs and implementations, all of which could bereferenced in any suitable timeframe.

Any of the memory items discussed herein should be construed as beingencompassed within the broad term ‘memory.’ Similarly, any of thepotential processing elements, modules, and machines described hereinshould be construed as being encompassed within the broad term‘microprocessor’ or ‘processor.’ Furthermore, in various embodiments,the processors, memories, network cards, buses, storage devices, relatedperipherals, and other hardware elements described herein may berealized by a processor, memory, and other related devices configured bysoftware or firmware to emulate or virtualize the functions of thosehardware elements.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer, as non-limitingexamples. Additionally, a computer may be embedded in a device notgenerally regarded as a computer but with suitable processingcapabilities, including a personal digital assistant (PDA), a smartphone, a mobile phone, an iPad, or any other suitable portable or fixedelectronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that may be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that may be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audibleformats.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks or wired networks.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that performs particular tasks or implement particularabstract data types. Typically, the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that may be employed to program a computer or otherprocessor to implement various aspects as described above. Additionally,it should be appreciated that according to one aspect, one or morecomputer programs that when executed perform methods of the presentapplication need not reside on a single computer or processor, but maybe distributed in a modular fashion among a number of differentcomputers or processors to implement various aspects of the presentapplication.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

When implemented in software, the software code may be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Computer program logic implementing all or part of the functionalitydescribed herein is embodied in various forms, including, but in no waylimited to, a source code form, a computer executable form, a hardwaredescription form, and various intermediate forms (for example, maskworks, or forms generated by an assembler, compiler, linker, orlocator). In an example, source code includes a series of computerprogram instructions implemented in various programming languages, suchas an object code, an assembly language, or a high-level language suchas OpenCL, RTL, Verilog, VHDL, Fortran, C, C++, JAVA, or HTML for usewith various operating systems or operating environments. The sourcecode may define and use various data structures and communicationmessages. The source code may be in a computer executable form (e.g.,via an interpreter), or the source code may be converted (e.g., via atranslator, assembler, or compiler) into a computer executable form.

In some embodiments, any number of electrical circuits of the FIGURESmay be implemented on a board of an associated electronic device. Theboard can be a general circuit board that can hold various components ofthe internal electronic system of the electronic device and, further,provide connectors for other peripherals. More specifically, the boardcan provide the electrical connections by which the other components ofthe system can communicate electrically. Any suitable processors(inclusive of digital signal processors, microprocessors, supportingchipsets, etc.), memory elements, etc. can be suitably coupled to theboard based on particular configuration needs, processing demands,computer designs, etc.

Other components such as external storage, additional sensors,controllers for audio/video display, and peripheral devices may beattached to the board as plug-in cards, via cables, or integrated intothe board itself. In another example embodiment, the electrical circuitsof the FIGURES may be implemented as standalone modules (e.g., a devicewith associated components and circuitry configured to perform aspecific application or function) or implemented as plug-in modules intoapplication-specific hardware of electronic devices.

Note that with the numerous examples provided herein, interaction may bedescribed in terms of two, three, four, or more electrical components.However, this has been done for purposes of clarity and example only. Itshould be appreciated that the system can be consolidated in anysuitable manner. Along similar design alternatives, any of theillustrated components, modules, and elements of the FIGURES may becombined in various possible configurations, all of which are clearlywithin the broad scope of this disclosure.

In certain cases, it may be easier to describe one or more of thefunctionalities of a given set of flows by only referencing a limitednumber of electrical elements. It should be appreciated that theelectrical circuits of the FIGURES and its teachings are readilyscalable and can accommodate a large number of components, as well asmore complicated/sophisticated arrangements and configurations.Accordingly, the examples provided should not limit the scope or inhibitthe broad teachings of the electrical circuits as potentially applied toa myriad of other architectures.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

Interpretation of Terms

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms. Unless the context clearly requires otherwise, throughout thedescription and the claims:

“comprise,” “comprising,” and the like are to be construed in aninclusive sense, as opposed to an exclusive or exhaustive sense; that isto say, in the sense of “including, but not limited to”.

“connected,” “coupled,” or any variant thereof, means any connection orcoupling, either direct or indirect, between two or more elements; thecoupling or connection between the elements can be physical, logical, ora combination thereof.

“herein,” “above,” “below,” and words of similar import, when used todescribe this specification shall refer to this specification as a wholeand not to any particular portions of this specification.

“or,” in reference to a list of two or more items, covers all of thefollowing interpretations of the word: any of the items in the list, allof the items in the list, and any combination of the items in the list.

the singular forms “a”, “an” and “the” also include the meaning of anyappropriate plural forms.

Words that indicate directions such as “vertical”, “transverse”,“horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”,“outward”, “vertical”, “transverse”, “left”, “right”, “front”, “back”,“top”, “bottom”, “below”, “above”, “under”, and the like, used in thisdescription and any accompanying claims (where present) depend on thespecific orientation of the apparatus described and illustrated. Thesubject matter described herein may assume various alternativeorientations. Accordingly, these directional terms are not strictlydefined and should not be interpreted narrowly.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined.

Elements other than those specifically identified by the “and/or” clausemay optionally be present, whether related or unrelated to thoseelements specifically identified. Thus, as a non-limiting example, areference to “A and/or B”, when used in conjunction with open-endedlanguage such as “comprising” may refer, in one embodiment, to A only(optionally including elements other than B); in another embodiment, toB only (optionally including elements other than A); in yet anotherembodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified.

Thus, as a non-limiting example, “at least one of A and B” (or,equivalently, “at least one of A or B,” or, equivalently “at least oneof A and/or B”) may refer, in one embodiment, to at least one,optionally including more than one, A, with no B present (and optionallyincluding elements other than B); in another embodiment, to at leastone, optionally including more than one, B, with no A present (andoptionally including elements other than A); in yet another embodiment,to at least one, optionally including more than one, A, and at leastone, optionally including more than one, B (and optionally includingother elements); etc.

As used herein, the term “between” is to be inclusive unless indicatedotherwise. For example, “between A and B” includes A and B unlessindicated otherwise.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

Numerous other changes, substitutions, variations, alterations, andmodifications may be ascertained to one skilled in the art and it isintended that the present disclosure encompass all such changes,substitutions, variations, alterations, and modifications as fallingwithin the scope of the appended claims.

In order to assist the United States Patent and Trademark Office (USPTO)and, additionally, any readers of any patent issued on this applicationin interpreting the claims appended hereto, Applicant wishes to notethat the Applicant: (a) does not intend any of the appended claims toinvoke 35 U.S.C. § 112(f) as it exists on the date of the filing hereofunless the words “means for” or “steps for” are specifically used in theparticular claims; and (b) does not intend, by any statement in thedisclosure, to limit this disclosure in any way that is not otherwisereflected in the appended claims.

The present invention should therefore not be considered limited to theparticular embodiments described above. Various modifications,equivalent processes, as well as numerous structures to which thepresent invention may be applicable, will be readily apparent to thoseskilled in the art to which the present invention is directed uponreview of the present disclosure.

What is claimed is:
 1. An apparatus for making gain independentreference channel measurements, the apparatus comprising: a firstcircuit configured to measure a stimulus passing through a testingobject; a first amplifier; a second circuit configured to measure astimulus which has not passed through the test volume; a secondamplifier; and a switching circuit in electrical communication with thefirst circuit, second circuit, first amplifier and second amplifier, theswitching circuit configured to change between: a first mode whereinfirst circuit is in electrical communication with the first amplifierand the second circuit is in electrical communication with the secondamplifier; and a second mode wherein first circuit is in electricalcommunication with the second amplifier and the second circuit is inelectrical communication with the first amplifier.
 2. The apparatusaccording to claim 1 wherein the first circuit comprises a sensor. 3.The apparatus according to claim 2 wherein the sensor is aphotodetector.
 4. The apparatus according to claim 3 wherein the secondcircuit comprises a photodetector.
 5. The apparatus according to claim 4further comprising a light source which produces the stimulus.
 6. Theapparatus according to claim 5 wherein the light source is an LED. 7.The apparatus according to claim 6 further comprising a current source.8. The apparatus according to claim 7 wherein the current source isconfigured to produce a predetermined waveform in both the first andsecond mode.
 9. The apparatus according to claim 8 further comprising ananalog front end (AFE) configured to compare output from the first andsecond gain amplifiers in the first and second modes.
 10. The apparatusaccording to claim 1 wherein the first circuit comprises a transducer.11. The apparatus according to claim 1 wherein the testing object is adevice under test which has an impedance to be measure by the stimuli.12. The apparatus according to claim 1 wherein the testing object is atesting volume.
 13. The apparatus according to claim 1 wherein thetesting object is a subject.
 14. A method for making gain independentreference channel measurements, the method comprising: measuring at afirst circuit a stimulus passing through a testing object; measuring ata second circuit a stimulus which has not passed through the testvolume; receiving at a switch circuit the measurements from the firstand second circuits; in a first mode, outputting from the switch circuitthe first and second measurement to a first and second amplifier,respectively; switching the switch circuit; and in a second mode,outputting from the switch circuit the first and second measurement tothe second and first amplifier, respectively.
 15. The method accordingto claim 14 further comprising amplifying the first and secondmeasurement in the first and second amplifiers, respectively, during thefirst mode.
 16. The method according to claim 15 further comprisingamplifying the first and second measurement in the second and firstamplifiers, respectively, during the second mode.
 17. The methodaccording to claim 16 further comprising calculating a ratio based on atleast amplified measurements in during the first and second modes. 18.The method according to claim 17 further comprising determining thepresence of a predetermined species based on the calculated ratio. 19.The method according to claim 18 wherein the species are smokeparticles.
 20. The method according to claim 18 wherein the species is abiochemical.